The invention relates to a retrofitting process of a band product treatment unit, in particular an acid pickling unit for metallic bands.
The unit also covers a unit obtained by the implementation of the process.
In units dedicated to the production of a laminated metallic band, for example of sheet or tinplate, it is customary to let the product pass through a pickling line comprising, normally, a series of pickling tubs filled with an acid solution, for instance hydrochlonic acid, and in which the metallic band is caused to run continuously, wherein the band unwinds from a reel, passing successively through each of the tubs and, after rinsing, is rewound at the output of the unit.
Such units are well-known and do not call for any detailed descriptions.
In older units, it is customary to use relatively deep tubs, located one after the other, inside which the band forms a loop plunging inside the liquid between two band passage thresholds placed above the level of liquid at both ends, respectively upstream and downstream of each tub.
Since the solution becomes loaded with oxides and impurities as the treatment progresses, the solution must be regenerated in order to maintain the desirable acid concentration. Usually, the concentrated solution is injected into the last tub and it overflows, quite simply, into the previous tub and so on and so forth, so that the band passes in succession through tubs whose concentration increases from upstream to downstream.
For some years, technology has evolved and, at the moment, we prefer to use flat or semi-flat tubs, which contain a smaller quantity of liquid and enable quick drainage should the line stop.
In this case, each tub is associated with a cistern or recirculation vessel located beside the line and which enables emptying the tub quickly and which makes maintaining the bath at the requested temperature and concentration easier.
Many pickling lines still in operation and built, in particular, before the Eighties, are fitted with deep tubs and it is interesting, to enhance the performances of the pickling line, to change the existing tubs with new type tubs, flat or semi-flat ones. Such an operation requires, however, prolonged shutdown of the line.
Moreover, the new pickling tubs, which contain a reduced quantity of liquid, must be associated with recirculation vessels, which, most often, do not exist in the former lines. Still, it is advisable not to bring too far-reaching retrofitting procedures to the whole plant and it is often difficult to find sufficient room in the vicinity of the line, to accommodate such a vessel.